Abstract:

The present invention relates to a method of manufacturing a magnetic
recording medium wherein the magnetic layer coating liquid comprising a
ferromagnetic powder having an average particle size of 10 to 40 nm and a
moisture content of 0.3 to 3.0 weight percent; a binder (a) comprising
0.2 to 0.7 meq/g of at least one polar group selected from the group
consisting of --SO3M, --OSO3M, --PO(OM)2, --OPO(OM)2,
and COOM (M denotes a hydrogen atom or the like) and having a weight
average molecular weight of 20,000 to 200,000, and/or (b) comprising 0.5
to 5 meq/g of at least one polar group selected from the group consisting
of --CONR1R2, --NR1R2, and
--NR+R1R2R3 (wherein R1, R2, and R3
each independently denote a hydrogen atom or the like) and having a
weight average molecular weight of 20,000 to 200,000; and a compound
comprising at least one carboxyl group and/or hydroxyl group per
molecule.

Claims:

1. A method of manufacturing a magnetic recording medium
comprising:coating a magnetic layer coating liquid on a nonmagnetic
support and drying the magnetic layer coating liquid to form a magnetic
layer, whereinthe magnetic layer coating liquid comprises components A, B
and C.Component A: A ferromagnetic powder having an average particle size
ranging from 10 to 40 nm and having a moisture content ranging from 0.3
to 3.0 weight percent;Component B: a binder (a) comprising 0.2 to 0.7
meq/g of at least one polar group selected from the group consisting of
--SO3M, --OSO3M, --PO(OM)2, --OPO(OM)2, and COOM,
wherein M denotes a hydrogen atom, alkali metal, or ammonium, and having
a weight average molecular weight ranging from 20,000 to 200,000, and/or
(b) comprising 0.5 to 5 meq/g of at least one polar group selected from
the group consisting of --CONR1R2, --NR1R2, and
--N+R1R2R3, wherein R1, R2, and R3
each independently denote a hydrogen atom or an alkyl group, and having a
weight average molecular weight ranging from 20,000 to 200,000;
andComponent C: a compound comprising at least one carboxyl group and/or
hydroxyl group per molecule.

2. The method of manufacturing a magnetic recording medium according to
claim 1, which comprises preparing the magnetic layer coating liquid by
simultaneously mixing components A, B, and C, or by mixing components A
and C to obtain a mixture and mixing component B to the mixture.

3. The method of manufacturing a magnetic recording medium according to
claim 1, wherein component B is a binder (a) comprising 0.2 to 0.7 meq/g
of at least one polar group selected from the group consisting of
--SO3M, --OSO3M, --PO(OM)2, --OPO(OM)2, and COOM,
wherein M denotes a hydrogen atom, alkali metal, or ammonium, and having
a weight average molecular weight ranging from 20,000 to 200,000.

4. The method of manufacturing a magnetic recording medium according to
claim 1, wherein the compound comprising at least one carboxyl group
and/or hydroxyl group per molecule is a cyclic compound.

5. The method of manufacturing a magnetic recording medium according to
claim 4, wherein the cyclic compound is at least one compound selected
from the group consisting of alicyclic compounds, aromatic compounds, and
heterocyclic compounds.

6. The method of manufacturing a magnetic recording medium according to
claim 4, wherein a cyclic structure comprised in the cyclic compound is
at least one selected from the group consisting of cyclohexane rings,
benzene rings, pyridine rings, and naphthalene rings.

7. The method of manufacturing a magnetic recording medium according to
claim 1, wherein the ferromagnetic powder is a hexagonal ferrite powder.

8. The method of manufacturing a magnetic recording medium according to
claim 1, wherein the binder is a polyurethane resin.

9. The method of manufacturing a magnetic recording medium according to
claim 1, which manufactures a magnetic recording medium comprising a
magnetic layer, the surface of which has a centerline average roughness
ranging from 1.0 to 3.0 nm.

10. A magnetic recording medium comprising a magnetic layer comprising a
ferromagnetic powder and a binder on a nonmagnetic support, manufactured
by the method according to claim 1.

11. The magnetic recording medium according to claim 10, wherein a
centerline average roughness of the magnetic layer surface ranges from
1.0 to 3.0 nm.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims the benefit of priority under 35 USC 119 to
Japanese Patent Application No. 2007-256815 filed on Sep. 28, 2007 and
Japanese Patent Application No. 2008-080264 filed on Mar. 26, 2008, which
are expressly incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to a method of manufacturing a
magnetic recording medium and a magnetic recording medium, and more
particularly, to a method of manufacturing a magnetic recording medium
that can exhibit good electromagnetic characteristics and inhibit head
grime.

[0004]2. Discussion of the Background

[0005]In recent years, means for rapidly transmitting information have
undergone marked development, making it possible to transmit data and
images comprising huge amounts of information. As data transmission
technology has improved, the need for higher density recording in the
recording media and recording and reproduction devices used to record,
reproduce, and store information has developed.

[0006]In addition to using microgranular magnetic materials, it is known
that dispersing microgranular magnetic materials to a high degree and
increasing the smoothness of the magnetic layer surface are effective
means of achieving good electromagnetic characteristics in the
high-density recording region. For example, Japanese Unexamined Patent
Publication (KOKAI) No. 2003-132531 or English language family member US
2003/0143323 A1 proposes increasing the quantity of polar groups in the
binder to within a prescribed range and controlling the moisture content
of the magnetic powder to within a prescribed range to increase
adsorption of binder to the magnetic material, prevent aggregation of
magnetic material, and enhance dispersion. The contents of these
applications re expressly incorporated herein by reference in their
entirety.

[0007]However, investigation conducted by the present inventors has
revealed that in a magnetic recording medium employing a binder in which
the quantity of polar groups has been increased and in which the surface
properties of the magnetic layer have been enhanced by the method
described in Japanese Unexamined Patent Publication (KOKAI) No.
2003-132531, although good electromagnetic characteristics are achieved
by enhancing dispersion of the magnetic material and thus enhancing the
surface properties of the magnetic layer obtained, accumulation of grime
on the head during running is quite pronounced. Head grime increases
noise and reduces the service lifetime of the head, and is thus desirably
minimized.

SUMMARY OF THE INVENTION

[0008]An aspect of the present invention provides for a magnetic recording
medium and a method of manufacturing a magnetic recording medium that can
exhibit good electromagnetic characteristics and inhibit head grime.

[0009]The present inventors conducted extensive research into achieving
the above manufacturing method and magnetic recording medium, resulting
in assuming that head grime is caused by the presence on the surface of
the magnetic layer of low-molecular-weight components derived from binder
in which the quantity of polar groups has been increased. The smoother
the surface of the magnetic layer, the greater the contact area becomes
between the magnetic layer and the head during running, and the greater
the amount is thought to be of adhesion to the head by the above
low-molecular-weight components present on the surface of the magnetic
layer.

[0010]Accordingly, based on the above assumptions, the present inventors
conducted research into achieving means of reducing the
low-molecular-weight components present on the surface of the magnetic
layer, first by employing a binder of relatively high molecular weight in
the magnetic layer. However, the results of this research by the present
inventors revealed that when a large quantity of polar groups was
introduced to enhance dispersibility, regardless of the
high-molecular-weight binder employed, the low-molecular-weight
components were still present on the surface of the magnetic layer. The
present inventors attributed this to the binder, with its heightened
adsorption to magnetic material resulting from the incorporation of a
large quantity of polar groups, coming into contact with active sites on
the surface of the magnetic material, the severing of polymer chains by
hydrolysis, and as a result, the release of low-molecular-weight
components.

[0011]The present inventors conducted further research based on the above
assumptions, discovering that, in addition to employing a
high-molecular-weight binder into which a large quantity of polar groups
had been incorporated, by adjusting the moisture content of the
ferromagnetic powder to within a prescribed range and employing a
compound comprising at least one carboxyl group and/or hydroxyl group per
molecule to form the magnetic layer, it was possible to increase the
dispersibility of the magnetic layer while inhibiting severing of the
high-molecular-weight binder, and as a result, to obtain a magnetic
recording medium having good electromagnetic characteristics in which
head grime was inhibited. The present invention was devised on that
basis.

[0012]An aspect of the present invention relates to a method of
manufacturing a magnetic recording medium comprising:

[0013]coating a magnetic layer coating liquid on a nonmagnetic support and
drying the magnetic layer coating liquid to form a magnetic layer,
wherein the magnetic layer coating liquid comprises components A, B and
C.

Component A: A ferromagnetic powder having an average particle size
ranging from 10 to 40 nm and having a moisture content ranging from 0.3
to 3.0 weight percent;Component B: a binder (a) comprising 0.2 to 0.7
meq/g of at least one polar group selected from the group consisting of
--SO3M, --OSO3M, --PO(OM)2, --OPO(OM)2, and COOM,
wherein M denotes a hydrogen atom, alkali metal, or ammonium, and having
a weight average molecular weight ranging from 20,000 to 200,000, and/or
(b) comprising 0.5 to 5 meq/g of at least one polar group selected from
the group consisting of --CONR1R2, --NR1R2, and
--N+R1R2R3, wherein R1, R2, and R3
each independently denote a hydrogen atom or an alkyl group, and having a
weight average molecular weight ranging from 20,000 to 200,000;
andComponent C: a compound comprising at least one carboxyl group and/or
hydroxyl group per molecule.

[0014]The above method may comprise preparing the magnetic layer coating
liquid by simultaneously mixing components A, B, and C, or by mixing
components A and C to obtain a mixture and mixing component B to the
mixture.

[0015]Component B may be the binder (a) comprising 0.2 to 0.7 meq/g of at
least one polar group selected from the group consisting of --SO3M,
--OSO3M, --PO(OM)2, --OPO(OM)2, and COOM, wherein M
denotes a hydrogen atom, alkali metal, or ammonium, and having a weight
average molecular weight ranging from 20,000 to 200,000.

[0016]The compound comprising at least one carboxyl group and/or hydroxyl
group per molecule may be a cyclic compound.

[0017]The cyclic compound may be at least one compound selected from the
group consisting of alicyclic compounds, aromatic compounds, and
heterocyclic compounds.

[0018]The cyclic structure comprised in the cyclic compound may be at
least one selected from the group consisting of cyclohexane rings,
benzene rings, pyridine rings, and naphthalene rings.

[0019]The above ferromagnetic powder may be a hexagonal ferrite powder.

[0020]The binder may be a polyurethane resin.

[0021]By the above method, a magnetic recording medium comprising a
magnetic layer, the surface of which has a centerline average roughness
ranging from 1.0 to 3.0 nm may be manufactured.

[0022]A further aspect of the present invention relates to a magnetic
recording medium comprising a magnetic layer comprising a ferromagnetic
powder and a binder on a nonmagnetic support, manufactured by the above
method.

[0023]The centerline average roughness of the magnetic layer surface may
range from 1.0 to 3.0 nm.

[0024]The present invention can provide a magnetic recording medium for
high-density recording that can exhibit good electromagnetic
characteristics and inhibit head grime.

[0025]Other exemplary embodiments and advantages of the present invention
may be ascertained by reviewing the present disclosure.

DETAILED DESCRIPTIONS OF THE EMBODIMENTS

[0026]The following preferred specific embodiments are, therefore, to be
construed as merely illustrative, and non-limiting to the remainder of
the disclosure in any way whatsoever. In this regard, no attempt is made
to show structural details of the present invention in more detail than
is necessary for fundamental understanding of the present invention; the
description taken with the drawings making apparent to those skilled in
the art how several forms of the present invention may be embodied in
practice.

Method of Manufacturing Magnetic Recording Medium

[0027]The present invention relates to a method of manufacturing a
magnetic recording medium comprising coating a magnetic layer coating
liquid on a nonmagnetic support and drying the magnetic layer coating
liquid to form a magnetic layer. In the above method, the magnetic layer
coating liquid comprises components A, B and C below.

[0028]Component A: A ferromagnetic powder having an average particle size
ranging from 10 to 40 nm and having a moisture content ranging from 0.3
to 3.0 weight percent;

[0029]Component B: a binder (a) comprising 0.2 to 0.7 meq/g of at least
one polar group selected from the group consisting of --SO3M,
--OSO3M, --PO(OM)2, --OPO(OM)2, and COOM, wherein M
denotes a hydrogen atom, alkali metal, or ammonium, and having a weight
average molecular weight ranging from 20,000 to 200,000, and/or (b)
comprising 0.5 to 5 meq/g of at least one polar group selected from the
group consisting of --CONR1R2, --NR1R2, and
--N+R1R2R3, wherein R1, R2, and R3
each independently denote a hydrogen atom or an alkyl group) and having a
weight average molecular weight ranging from 20,000 to 200,000; and

[0030]Component C: a compound comprising at least one carboxyl group
and/or hydroxyl group per molecule.

[0031]In the method of manufacturing a magnetic recording medium of the
present invention, the use of a ferromagnetic powder in the form of a
microgranular magnetic material (component A) having an average particle
size ranging from 10 to 40 nm with components B and C can increase the
smoothness of the surface of the magnetic layer, thereby yielding a
magnetic recording medium having good electromagnetic characteristics.
More specifically, to increase adsorption of the binder to the magnetic
material and enhance dispersibility, a prescribed quantity of the above
polar groups is incorporated into the binder of the magnetic layer and
the moisture content of the ferromagnetic powder is kept to a prescribed
quantity. Thus, the dispersibility of the ferromagnetic powder can be
enhanced and the smoothness of the surface of the magnetic layer can be
increased. Further, by incorporating a binder of relatively high
molecular weight in the form of a binder with a weight average molecular
weight of 20,000 to 200,000 into the magnetic layer with the
above-described compounds, it is possible to inhibit the accumulation of
grime on the head during running by means of the magnetic layer having
the above-stated smoothness. This is thought to occur for the following
two reasons:

(1) Even when free binder that has not adhered to the magnetic material is
present in the outer portion of the magnetic layer, the relatively high
molecular weight of the binder can cause it to tend not to adhere to the
head, so it may not cause head grime.(2) The low-molecular-weight
components derived from binder are thought to be produced by hydrolysis
of the binder due to the binder coming into contact with active sites on
the surface of the magnetic material. Since a binder into which a
relatively large number of polar groups has been incorporated as set
forth above has a high degree of adsorption to magnetic material, the
ratio of contact between binder and active sites on the surface of the
magnetic layer is high. By contrast, since the above described compound
(component C) has high adsorptivity to magnetic material, when employed
as a component in the magnetic layer, it is thought to adhere to the
surface of the magnetic material and deactivate active sites on the
surface of the magnetic material. The generation of low-molecular-weight
components by severing of the polymer chains by hydrolysis of the binder
is thought to be thus inhibited.

[0032]The method of manufacturing a magnetic recording medium of the
present invention will be described in detail below.

Component C

[0033]The magnetic layer coating liquid comprises at least one compound
(component C) comprising at least one carboxyl group and/or hydroxyl
group per molecule. Achieving good dispersion of ferromagnetic powder
requires preventing aggregation between ferromagnetic powders. Preventing
aggregation between ferromagnetic powders requires causing the binder to
adsorb to the surface of the ferromagnetic powder. In this process,
causing a compound comprising at least one carboxyl group and/or hydroxyl
group per molecule to adsorb to the ferromagnetic powder can prevent
aggregation between ferromagnetic powders and enhance the dispersion of
the ferromagnetic powders. Further, the compound comprising the carboxyl
group and/or hydroxyl group can have high adsorptivity to the
ferromagnetic powder and function as a surface modifying agent on the
ferromagnetic powder. Thus, it is possible to inhibit the generation of a
large quantity of low-molecular-weight components derived from component
B due to contact between ferromagnetic powder (component A) and binder
(component B).

[0034]The above compound can comprise just a carboxyl group or a hydroxyl
group, or may comprise both. The number of these groups per molecule of
the compound is at least 1, preferably 1 to 5, and more preferably, 1 to
3.

[0035]So long as the above compound (so-called "surface-modifying agent")
comprises at least 1 carboxyl group and/or hydroxyl group per molecule,
it may be a cyclic compound or chain compound, but a cyclic compound is
desirable.

[0036]The cyclic structure contained in the above cyclic compound may be
that of an aliphatic ring, aromatic ring, or hetero ring. That is,
examples of the above cyclic compound are one or more members selected
from the group consisting of alicyclic compounds, aromatic compounds, and
heterocyclic compounds. The cyclic structure may be in the form of a
single ring or a condensed ring. There may be one or more cyclic
structures contained in the molecule, and the structure may be one in
which different types of cyclic structures are linked by linking groups.
For example, the cyclic structure contained in the above cyclic compound
may suitably be one or more selected from the group consisting of
cyclohexane rings, benzene rings, pyridine rings, and naphthalene rings.

[0037]When the cyclic compound is an alicyclic compound, the cyclic
structure contained is, for example, an optionally condensed aliphatic
ring having 5 to 30 carbon atoms, desirably an optionally condensed
aliphatic ring having 5 to 10 carbon atoms, and preferably, a cyclohexane
ring.

[0038]When the cyclic compound is an aromatic compound, the aromatic ring
contained is desirably a five-membered ring, six-membered ring,
seven-membered ring, or a ring formed by the condensation of a
combination thereof, preferably a five-membered ring or six-membered
ring, and more preferably, a six-membered ring. Specific examples are
benzene rings, naphthalene rings, anthracene rings, and phenanthrene
rings. Of these, benzene rings and naphthalene rings are desirable.

[0042]Component C can be readily synthesized by known methods and may be
commercially available.

[0043]The content of component C in the magnetic layer can be suitably
set, but is desirably 0.1 to 10 weight parts, preferably 0.5 to 10 weight
parts, and more preferably, 1 to 8 weight parts per 100 weight parts of
ferromagnetic powder. By keeping the content of component C less than or
equal to the upper limit of the above range, plasticizing and peeling of
the film can be inhibited. Additionally, by keeping the content of
component C greater than or equal to the lower limit of the above range,
head grime can be prevented.

Binder (Component B)

[0044]The binder (component B) contained in the magnetic layer coating
liquid is a binder (a) comprising 0.2 to 0.7 meq/g of at least one polar
group selected from the group consisting of --SO3M, --OSO3M,
--PO(OM)2, --OPO(OM)2, and COOM (wherein M denotes a hydrogen
atom, alkali metal, or ammonium) and having a weight average molecular
weight ranging from 20,000 to 200,000, and/or (b) comprising 0.5 to 5
meq/g of at least one polar group selected from the group consisting of
--CONR1R2, --NR1R2, and
--N+R1R2R3 (wherein R1, R2, and R3
each independently denote a hydrogen atom or an alkyl group) and having a
weight average molecular weight ranging from 20,000 to 200,000. That is,
the binder may meet the requirements of either (a) or (b), or both. The
binder desirably satisfies the requirements of at least (a), and is
preferably (a). Any one from among --SO3M, --OSO3M,
--PO(OM)2, and COOM is desirable as the polar group in (a). The
above alkyl group desirably has 1 to 18 carbon atoms, and may have a
linear or branched structure. The content of the polar group in the
binder (a) is 0.2 to 0.7 meq/g, desirably 0.25 to 0.6 meq/g, and
preferably, 0.3 to 0.5 meq/g. The content of the polar group in (b) is
0.5 to 5 meq/g, desirably 1 to 4 meq/g, and preferably, 1.5 to 3.5 meq/g.
When the content of the polar group falls outside the above range, it
becomes difficult to increase the dispersibility of the magnetic material
and achieve a magnetic layer of good surface smoothness. One or more
types of the above polar groups may be incorporated. The content of the
polar groups in (a) and (b) refers to the combined content when multiple
types of polar group are present. The polar group can be incorporated in
desired quantity into the binder by addition polymerization or
copolymerization, for example.

[0045]The weight average molecular weight of the binder falls within a
range of 20,000 to 200,000. When the weight average molecular weight is
less than 20,000, head grime becomes pronounced. This is thought to be
due to an increase in the quantity of low-molecular-weight component in
the outer portion of the magnetic layer. When the weight average
molecular weight exceeds 200,000, dispersibility diminishes and it
becomes difficult to obtain good electromagnetic characteristics. The
weight average molecular weight is desirably 30,000 to 180,000,
preferably 50,000 to 150,000.

[0046]So long as the binder satisfies the conditions of (a) and/or (b)
above and has a weight average molecular weight within the above-stated
range, the structure and the like of the binder are not specifically
limited. Conventionally known thermoplastic resins, thermosetting resins,
reactive resins, polymers, mixtures thereof, and the like can be
employed. Examples are: polymers and copolymers comprising structural
units in the form of vinyl chloride, vinyl acetate, vinyl alcohol, maleic
acid, acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile,
methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl
butyral, vinyl acetal, or vinyl ether; polyurethane resins; and various
rubber-based resins. Examples of thermosetting resins and reactive resins
are: phenol resin, epoxy resin, polyurethane cured resins, urea resins,
melamine resins, alkyd resins, acrylic reactive resins, formaldehyde
resins, silicone resins, epoxy-polyamide resins, mixtures of a polyester
resin and an isocyanate prepolymer, mixtures of a polyester polyol and a
polyisocyanate, and mixtures of polyurethane and a polyisocyanate. These
resins are described in detail in Handbook of plastics published by
Asakura Shoten, which is expressly incorporated herein by reference in
its entirety. It is also possible to employ known electron beam-cured
resins in each layer. Examples and manufacturing methods of such resins
are described in Japanese Unexamined Patent Publication (KOKAI) Showa No.
62-256219, which is expressly incorporated herein by reference in its
entirety. The above-listed resins may be used singly or in combination.
Those comprising polyurethane are desirable. Examples of suitable resins
are combinations of a polyurethane resin with one or more selected from
among vinyl chloride resin, vinyl chloride-vinyl acetate copolymers,
vinyl chloride-vinyl acetate-vinyl alcohol copolymers, and vinyl
chloride-vinyl acetate-maleic anhydride copolymers; and combinations of
polyisocyanate with the same. In the manufacturing method of the present
invention, particularly in a magnetic recording medium in which
polyurethane resin is employed, it is possible to effectively inhibit
head grime.

[0048]The above binder can be synthesized by known methods. Further,
commercial products can be employed as they are, or desirable quantities
of polar groups can be incorporated for use.

[0049]As set forth below, the magnetic recording medium that is
manufactured by the manufacturing method of the present invention may
comprise a nonmagnetic layer comprising a nonmagnetic powder and a binder
between the magnetic layer and the nonmagnetic support. Examples of
binders that are suitable for use in the nonmagnetic layer are the
binders that are suitable for use in the magnetic layer. Binders that are
employed in common magnetic layers may also be employed.

[0050]The above binder is employed, for example, in a range of 5 to 50
weight percent, desirably in a range of 10 to 30 weight percent, relative
to the nonmagnetic powder employed in the nonmagnetic layer or
ferromagnetic powder employed in the magnetic layer. Vinyl chloride resin
is desirably combined for use in a range of 5 to 30 weight percent when
employed. Polyurethane resin is desirably combined for use in a range of
2 to 20 weight percent when employed. And polyisocyanate is desirably
combined for use in a range of 2 to 20 weight percent when employed.
However, when a small amount of dechlorination causes head corrosion, it
is also possible to employ polyurethane alone, or employ polyurethane and
isocyanate alone. When polyurethane is employed, a glass transition
temperature of -50 to 150° C., preferably 0 to 100° C., an
elongation at break of 100 to 2,000 percent, a stress at break of 0.05 to
10 kg/mm2 (approximately 0.49 to 98 MPa), and a yield point of 0.05
to 10 kg/mm2 (approximately 0.49 to 98 MPa) are desirable.

[0051]Examples of polyisocyanates are tolylene diisocyanate,
4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene
diisocyanate, napthylene-1,5-diisocyanate, o-toluidine diisocyanate,
isophorone diisocyanate, triphenylmethane triisocyanate, and other
isocyanates; products of these isocyanates and polyalcohols;
polyisocyanates produced by condensation of isocyanates; and the like.
These isocyanates are commercially available under the following trade
names, for example: Coronate L, Coronate HL, Coronate 2030, Coronate
2031, Millionate MR and Millionate MTL manufactured by Nippon
Polyurethane Industry Co. Ltd.; Takenate D-102, Takenate D-110N, Takenate
D-200 and Takenate D-202 manufactured by Takeda Chemical Industries Co.,
Ltd.; and Desmodule L, Desmodule IL, Desmodule N and Desmodule HL
manufactured by Sumitomo Bayer Co., Ltd. They can be used in each layer
singly or in combinations of two or more by exploiting differences in
curing reactivity.

[0052]As set forth above, the use of component C is thought to reduce head
grime because when a high-molecular-weight binder is employed, component
C can deactivate active sites on the surface of the ferromagnetic powder,
preventing the binder from undergoing hydrolysis and the like to produce
low-molecular-weight components, thereby reducing the quantity of
low-molecular-weight components causing head grime on the magnetic layer
surface. The weight average molecular weight of the binder (resin
component) can be measured by the following method.

[0053](Method of Measuring the Weight Average Molecular Weight of a Resin
Component)

[0054]The binder is evaluated by gel permeation chromatography (GPC). The
weight average molecular weight of the resin component is the value
obtained by conversion based on standard polystyrene samples using
dimethyl formamide (DMF) solvent.

Surface Roughness of the Magnetic Layer

[0055]The surface roughness of the magnetic layer of the magnetic
recording medium manufactured by the method of manufacturing a magnetic
recording medium of the present invention desirably ranges from 1.0 to
3.0 nm as a centerline average roughness. When the centerline average
roughness of the magnetic layer is equal to or lower than 3.0 nm, better
electromagnetic characteristics can be achieved, and when equal to or
greater than 1.0 nm, running stability can increase. The centerline
average roughness of the magnetic layer is desirably 1.5 to 3.0 nm,
preferably 1.5 to 2.5 nm. Use of a magnetic layer coating liquid
comprising components A, B, and C permits the formation of a magnetic
layer of good surface smoothness. The surface smoothness of the magnetic
layer can also be controlled through the particle size of the
ferromagnetic powder, the dispersion conditions of the magnetic layer
coating liquid, calendering conditions, adjustment of the quantity of
filler in the nonmagnetic support, the use of an undercoating layer for
smoothness, and the like.

Ferromagnetic Powder (Component A)

[0056]Hexagonal ferrite powder and ferromagnetic metal powder can be
employed as the ferromagnetic powder (component A) contained in the
magnetic layer coating liquid. Hexagonal ferrite powder is desirably
employed. When the length of the signal recording region approaches the
size of the magnetic material contained in the magnetic layer, it becomes
impossible to create a distinct magnetization transition state,
essentially precluding recording. Thus, the shorter the recording
wavelength becomes, the smaller the magnetic material should be. To
achieve good electromagnetic characteristics in the present invention,
ferromagnetic powder with an average particle size of 10 to 40 nm is
employed. When the average particle size is less than 10 nm, it becomes
difficult to disperse individual particles. This means that it becomes
difficult to cover individual magnetic particles with binder. In this
case, the surface of several aggregated magnetic particles is covered
with binder, and thus there will be aggregates in which no binder is
present between the magnetic particles, weakening the bonds between
magnetic particles. This is thought to decrease the coating strength of
the magnetic layer. When the average particle size exceeds 40 nm, it
becomes difficult to achieve good electromagnetic characteristics. The
average particle size is desirably 15 to 40 nm, preferably 15 to 30 nm.

[0057]The average particle size of the ferromagnetic powder can be
measured by the following method.

[0058]Ferromagnetic powder is photographed at a magnification of
100,000-fold with a model H-9000 transmission electron microscope made by
Hitachi, and the photographs are printed on photographic paper at a total
magnification of 500,000 to obtain particle photographs. Target magnetic
particles are selected in the particle photographs, the outlines of the
particles are traced with a digitizer, and the particle size is measured
with KS-400 image analysis software from Carl Zeiss. The size of 500
particles is measured. The average value of the size of the particles
measured by the above-described method is then adopted as the average
particle size of the ferromagnetic powder.

[0059]The size of a powder such as the magnetic material (referred to as
the "powder size" hereinafter) in the present invention is denoted: (1)
by the length of the major axis constituting the powder, that is, the
major axis length, when the powder is acicular, spindle-shaped, or
columnar in shape (and the height is greater than the maximum major
diameter of the bottom surface); (2) by the maximum major diameter of the
tabular surface or bottom surface when the powder is tabular or columnar
in shape (and the thickness or height is smaller than the maximum major
diameter of the tabular surface or bottom surface); and (3) by the
diameter of an equivalent circle when the powder is spherical,
polyhedral, or of unspecified shape and the major axis constituting the
powder cannot be specified based on shape. The "diameter of an equivalent
circle" refers to that obtained by the circular projection method.

[0060]The average powder size of the powder is the arithmetic average of
the above powder size and is calculated by measuring five hundred primary
particles in the above-described method. The term "primary particle"
refers to a nonaggregated, independent particle.

[0061]The average acicular ratio of the powder refers to the arithmetic
average of the value of the (major axis length/minor axis length) of each
powder, obtained by measuring the length of the minor axis of the powder
in the above measurement, that is, the minor axis length. The term "minor
axis length" means the length of the minor axis constituting a powder for
a powder size of definition (1) above, and refers to the thickness or
height for definition (2) above. For (3) above, the (major axis
length/minor axis length) can be deemed for the sake of convenience to be
1, since there is no difference between the major and minor axes.

[0062]When the shape of the powder is specified, for example, as in
particle size definition (1) above, the average particle size refers to
the average major axis length. For definition (2) above, the average
particle size refers to the average plate diameter, with the arithmetic
average of (maximum major diameter/thickness or height) being referred to
as the average plate ratio. For definition (3), the average particle size
refers to the average diameter (also called the average particle
diameter). In the measurement of powder size, the standard
deviation/average value, expressed as a percentage, is defined as the
coefficient of variation.

[0063]The moisture content of the ferromagnetic powder contained in the
magnetic layer coating liquid employed in the method of manufacturing a
magnetic recording medium of the present invention is 0.3 to 3 weight
percent, desirably 0.5 to 1.5 weight percent, and preferably, 0.8 to 1.5
weight percent. Keeping the moisture content to within the above range
cab optimize adsorption of the binder (component B) containing a
prescribed quantity of polar groups to the magnetic material and enhance
dispersibility, making it possible to achieve a magnetic recording medium
exhibiting a high S/N ratio. A moisture content in the ferromagnetic
powder of less than 0.3 weight percent is undesirable in that the binder
does not adsorb adequately to reduce dispersibility. A moisture content
exceeding 3 weight percent is undesirable in that an excessive reaction
takes place between the binder and the curing agent, such as
polyisocyanate, in the magnetic layer coating liquid, raising the
viscosity of the magnetic layer coating liquid. The moisture content can
be adjusted by drying or adding water after manufacturing the magnetic
material. The moisture content can be measured by the Karl Fischer's
method. The Karl Fischer's method of measuring moisture content can be
employed as set forth below.

[0064]The temperature of a vaporizer is set to 120° C. A carrier
gas (N2) is passed through at a flow rate of 300 mL/min. About 300
mg of sample is precisely weighed out and a trace water meter (CA-05)
with vaporizer (VA-05) made by Mitsubishi Chemicals (Ltd.) is employed to
obtain the absolute moisture content. The moisture content of the sample
is then calculated from the following equation:

[0068]As the hexagonal ferrite powder, those having an average plate
diameter ranging from 10 to 40 nm are employed. The average plate
diameter preferably ranges from 15 to 40 nm, more preferably 15 to 30 nm.

[0069]An average plate ratio [arithmetic average of (plate diameter/plate
thickness)] preferably ranges from 1 to 15, more preferably 1 to 7. When
the average plate diameter ranges from 1 to 15, adequate orientation can
be achieved while maintaining high filling property, as well as increased
noise due to stacking between particles can be suppressed. The specific
surface area by BET method (SBET) within the above particle size
range is preferably equal to or higher than 40 m2/g, more preferably
40 to 200 m2/g, and particularly preferably, 60 to 100 m2/g.

[0070]Narrow distributions of particle plate diameter and plate thickness
of the hexagonal ferrite powder are normally good. About 500 particles
can be randomly measured in a transmission electron microscope (TEM)
photograph of particles to measure the particle plate diameter and plate
thickness, as set forth above. The distributions of particle plate
diameter and plate thickness are often not a normal distribution.
However, when expressed as the standard deviation to the average size,
σ/average size may be 0.1 to 1.0. The particle producing reaction
system is rendered as uniform as possible and the particles produced are
subjected to a distribution-enhancing treatment to achieve a narrow
particle size distribution. For example, methods such as selectively
dissolving ultrafine particles in an acid solution by dissolution are
known.

[0071]A coercivity (Hc) of the hexagonal ferrite powder of about 143.3 to
318.5 kA/m (approximately 1800 to 4,000 Oe) can normally be achieved. The
coercivity (Hc) of the hexagonal ferrite powder preferably ranges from
167.2 to 294.5 kA/m (approximately 2,100 to 3,700 Oe), more preferably
199.0 to 278.6 kA/m (approximately 2,500 to 3,500 Oe). The coercivity
(Hc) can be controlled by particle size (plate diameter and plate
thickness), the types and quantities of elements contained, substitution
sites of the element, the particle producing reaction conditions, and the
like.

[0072]The φm of the magnetic layer can be controlled by the
saturation magnetization (σs) of the hexagonal ferrite powder.
The higher saturation magnetization (σs) is generally
preferred, however, it tends to decrease with decreasing particle size.
The saturation magnetization (σs) of the hexagonal ferrite
powder can be selected based on the desired φm, and preferably
30 to 80 Am2/kg (30 to 80 emu/g). Known methods of improving
saturation magnetization (σs) are combining spinel ferrite
with magnetoplumbite ferrite, selection of the type and quantity of
elements incorporated, and the like. It is also possible to employ W-type
hexagonal ferrite. When dispersing the hexagonal ferrite powder, the
surface of the hexagonal ferrite powder can be processed with a substance
suited to a dispersion medium and a polymer. The pH of the hexagonal
ferrite powder is also important to dispersion. A pH of about 4 to 12 is
usually optimum for the dispersion medium and polymer. From the
perspective of the chemical stability and storage properties of the
medium, a pH of about 6 to 11 can be selected. Since moisture contained
in the hexagonal ferrite powder also affects dispersion, the
ferromagnetic powder having the above-described moisture content is
employed in the present invention.

[0073]Methods of manufacturing the hexagonal ferrite powder include: (1) a
vitrified crystallization method consisting of mixing into a desired
ferrite composition barium oxide, iron oxide, and a metal oxide
substituting for iron with a glass forming substance such as boron oxide;
melting the mixture; rapidly cooling the mixture to obtain an amorphous
material; reheating the amorphous material; and refining and comminuting
the product to obtain a barium ferrite crystal powder; (2) a hydrothermal
reaction method consisting of neutralizing a barium ferrite composition
metal salt solution with an alkali; removing the by-product; heating the
liquid phase to equal to or greater than 100° C.; and washing,
drying, and comminuting the product to obtain barium ferrite crystal
powder; and (3) a coprecipitation method consisting of neutralizing a
barium ferrite composition metal salt solution with an alkali; removing
the by-product; drying the product and processing it at equal to or less
than 1,100° C.; and comminuting the product to obtain barium
ferrite crystal powder. Any manufacturing method can be selected in the
present invention. As needed, the hexagonal ferrite powder can be surface
treated with Al, Si, P, or an oxide thereof. The quantity can be set to
0.1 to 10 weight percent of the hexagonal ferrite powder. When applying a
surface treatment, the quantity of a lubricant such as a fatty acid that
is adsorbed is desirably not greater than 100 mg/m2. The hexagonal
ferrite powder will sometimes contain inorganic ions such as soluble Na,
Ca, Fe, Ni, or Sr. These are desirably substantially not present, but
seldom affect characteristics at equal to or less than 200 ppm.

(ii) Ferromagnetic Metal Powder

[0074]The ferromagnetic metal powder employed in the magnetic layer is not
specifically limited, but preferably a ferromagnetic metal power
comprised primarily of α-Fe. In addition to prescribed atoms, the
following atoms can be contained in the ferromagnetic metal powder: Al,
Si, S, Sc, Ca, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W,
Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B and the
like. Particularly, incorporation of at least one of the following in
addition to α-Fe is desirable: Al, Si, Ca, Y, Ba, La, Nd, Co, Ni,
and B. Incorporation of at least one selected from the group consisting
of Co, Y and Al is particularly preferred. The Co content preferably
ranges from 0 to 40 atom percent, more preferably from 15 to 35 atom
percent, further preferably from 20 to 35 atom percent with respect to
Fe. The content of Y preferably ranges from 1.5 to 12 atom percent, more
preferably from 3 to 10 atom percent, further preferably from 4 to 9 atom
percent with respect to Fe. The A1 content preferably ranges from 1.5 to
12 atom percent, more preferably from 3 to 10 atom percent, further
preferably from 4 to 9 atom percent with respect to Fe.

[0075]These ferromagnetic metal powders may be pretreated prior to
dispersion with dispersing agents, lubricants, surfactants, antistatic
agents, and the like, described further below. Specific examples are
described in Japanese Examined Patent Publication (KOKOKU) Showa Nos.
44-14090, 45-18372, 47-22062, 47-22513, 46-28466, 46-38755, 47-4286,
47-12422, 47-17284, 47-18509, 47-18573, 39-10307, and 46-39639; and U.S.
Pat. Nos. 3,026,215, 3,031,341, 3,100,194, 3,242,005, and 3,389,014,
which are expressly incorporated herein by reference in their entirety.

[0076]The ferromagnetic metal powder may contain a small quantity of
hydroxide or oxide. Ferromagnetic metal powders obtained by known
manufacturing methods may be employed. The following are examples of
methods of manufacturing ferromagnetic metal powders: methods of
reduction with compound organic acid salts (chiefly oxalates) and
reducing gases such as hydrogen; methods of reducing iron oxide with a
reducing gas such as hydrogen to obtain Fe or Fe--Co particles or the
like; methods of thermal decomposition of metal carbonyl compounds;
methods of reduction by addition of a reducing agent such as sodium boron
hydride, hypophosphite, or hydrazine to an aqueous solution of
ferromagnetic metal; and methods of obtaining powder by vaporizing a
metal in a low-pressure inert gas. Any one from among the known method of
slow oxidation, that is, immersing the ferromagnetic metal powder thus
obtained in an organic solvent and drying it; the method of immersing the
ferromagnetic metal powder in an organic solvent, feeding in an
oxygen-containing gas to form a surface oxide film, and then conducting
drying; and the method of adjusting the partial pressures of oxygen gas
and an inert gas without employing an organic solvent to form a surface
oxide film, may be employed.

[0077]The specific surface area by BET method of the ferromagnetic metal
powder employed in the magnetic layer is preferably 45 to 100 m2/g,
more preferably 50 to 80 m2/g. At 45 m2/g and above, low noise
is achieved. At 100 m2/g and below, good surface properties are
achieved. The crystallite size of the ferromagnetic metal powder is
preferably 40 to 180 Angstroms, more preferably 40 to 150 Angstroms, and
still more preferably, 40 to 110 Angstroms. The major axis length of the
ferromagnetic metal powder ranges from 10 to 40 nm, preferably from 15 to
30 nm. The acicular ratio of the ferromagnetic metal powder is preferably
equal to or greater than 3 and equal to or less than 15, more preferably
equal to or greater than 3 and equal to or less than 12. The
σs of the ferromagnetic metal powder is preferably 80 to 180
Am2/kg, more preferably 80 to 150 Am2/kg, and still more
preferably, 80 to 120 Am2/kg. The coercivity of the ferromagnetic
powder is preferably 2,000 to 3,500 Oe, approximately 160 to 280 kA/m,
more preferably 2,200 to 3,000 Oe, approximately 176 to 240 kA/m.

[0078]As set forth above, the moisture content of the ferromagnetic metal
powder ranges from 0.3 to 3 weight percent. The moisture content of the
ferromagnetic metal powder is desirably optimized based on the type of
binder. The pH of the ferromagnetic metal powder is desirably optimized
depending on what is combined with the binder. A range of 4 to 12 can be
established, with 6 to 10 being preferred. As needed, the ferromagnetic
metal powder can be surface treated with Al, Si, P, or an oxide thereof.
The quantity can be set to 0.1 to 10 weight percent of the ferromagnetic
metal powder. When applying a surface treatment, the quantity of a
lubricant such as a fatty acid that is adsorbed is desirably not greater
than 100 mg/m2. The ferromagnetic metal powder will sometimes
contain inorganic ions such as soluble Na, Ca, Fe, Ni, or Sr. These are
desirably substantially not present, but seldom affect characteristics at
equal to or less than 200 ppm. The ferromagnetic metal powder employed in
the present invention desirably has few voids; the level is preferably
equal to or less than 20 volume percent, more preferably equal to or less
than 5 volume percent. As stated above, so long as the particle size
characteristics are satisfied, the ferromagnetic metal powder may be
acicular, rice grain-shaped, or spindle-shaped. The SFD of the
ferromagnetic metal powder itself is desirably low, with equal to or less
than 0.8 being preferred. The Hc distribution of the ferromagnetic metal
powder is desirably kept low. When the SFD is equal to or lower than 0.8,
good electromagnetic characteristics are achieved, output is high, and
magnetic inversion is sharp, with little peak shifting, in a manner
suited to high-density digital magnetic recording. To keep the Hc low,
the methods of improving the particle size distribution of goethite in
the ferromagnetic metal powder and preventing sintering may be employed.

[0079]In the manufacturing method of the present invention, known
techniques regarding binders, lubricants, dispersion agents, additives,
solvents, dispersion methods and the like for magnetic layer, nonmagnetic
layer and backcoat layer that is optionally provided can be suitably
applied. In particular, known techniques regarding the quantity and types
of binders, and quantity added and types of additives and dispersion
agents can be applied.

[0081]It is also possible to employ nonionic surfactants such as alkylene
oxide-based surfactants, glycerin-based surfactants, glycidol-based
surfactants and alkylphenolethylene oxide adducts; cationic surfactants
such as cyclic amines, ester amides, quaternary ammonium salts, hydantoin
derivatives, phosphoniums, and sulfoniums; anionic surfactants comprising
acid groups, such as carboxylic acid, sulfonic acid, phosphoric acid,
sulfuric ester groups, and phosphoric ester groups; and ampholytic
surfactants such as amino acids, amino sulfonic acids, sulfuric or
phosphoric esters of amino alcohols, and alkyl betaines. Details of these
surfactants are described in A Guide to Surfactants (published by Sangyo
Tosho K.K.), which is expressly incorporated herein by reference in its
entirety.

[0082]These lubricants, antistatic agents and the like need not be 100
percent pure and may contain impurities, such as isomers, unreacted
material, by-products, decomposition products, and oxides in addition to
the main components. These impurities are preferably comprised equal to
or less than 30 weight percent, and more preferably equal to or less than
10 weight percent.

[0084]Component C can serve as a dispersing agent, and can be added to a
nonmagnetic layer coating liquid. In the present invention, component C
can be employed together with other compounds having a
dispersion-improving effect. The dispersion agent suitable use together
with component C is preferably at least one selected from the group
consisting of alicyclic compounds, aromatic compounds, and heterocyclic
compounds. Among component C and cyclic compounds other than component C,
those suitable for use as a dispersion agent are: phenol, benzoic acid,
cyclohexanol, cyclohexane carboxylic acid, 1-naphthoic acid, catechol,
and structural isomers thereof, phthalic acid and structural isomers
thereof, cyclohexane dicarboxylic acid and structural isomers thereof,
4-tert-butylphenol and structural isomers thereof, 4-butylphenol and
structural isomers thereof, 4-hydroxypyridine and structural isomers
thereof, 4-tert-butylbenzoic acid and structural isomers thereof, and
niacin.

[0085]Carbon black may be added to the magnetic layer as needed. Examples
of types of carbon black that are suitable for use in the magnetic layer
are: furnace black for rubber, thermal for rubber, black for coloring,
and acetylene black. It is preferable that the specific surface area is 5
to 500 m2/g, the DBP oil absorption capacity is 10 to 400 ml/100 g,
the particle diameter is 5 to 300 nm, the pH is 2 to 10, the moisture
content is 0.1 to 10 percent, and the tap density is 0.1 to 1 g/ml.

[0086]Specific examples of types of carbon black employed are: BLACK
PEARLS 2000, 1300, 1000, 900, 905, 800, 700 and VULCAN XC-72 from Cabot
Corporation; #80, #60, #55, #50 and #35 manufactured by Asahi Carbon Co.,
Ltd.; #2400B, #2300, #900, #1000, #30, #40 and #10B from Mitsubishi
Chemical Corporation; CONDUCTEX SC, RAVEN 150, 50, 40, 15 and RAVEN MT-P
from Columbia Carbon Co., Ltd.; and Ketjen Black EC from Ketjen Black
International Co., Ltd. The carbon black employed may be surface-treated
with a dispersant or grafted with resin, or have a partially
graphite-treated surface. The carbon black may be dispersed in advance
into the binder prior to addition to the magnetic coating liquid. These
carbon blacks may be used singly or in combination. When employing carbon
black, the quantity preferably ranges from 0.1 to 30 weight percent with
respect to the weight of the magnetic material. In the magnetic layer,
carbon black can work to prevent static, reduce the coefficient of
friction, impart light-blocking properties, enhance film strength, and
the like; the properties vary with the type of carbon black employed.
Accordingly, the type, quantity, and combination of carbon blacks
employed in the present invention may be determined separately for the
magnetic layer and the nonmagnetic layer based on the objective and the
various characteristics stated above, such as particle size, oil
absorption capacity, electrical conductivity, and pH, and be optimized
for each layer. For example, the Carbon Black Handbook compiled by the
Carbon Black Association, which is expressly incorporated herein by
reference in its entirety, may be consulted for types of carbon black
suitable for use in the magnetic layer.

Abrasives

[0087]Known materials chiefly having a Mohs' hardness of equal to or
greater than 6 may be employed either singly or in combination as
abrasives. These include: α-alumina, β-alumina, silicon
carbide, chromium oxide, cerium oxide, α-iron oxide, corundum,
synthetic diamond, silicon nitride, titanium carbide, titanium oxide,
silicon dioxide, and boron nitride. Complexes of these abrasives
(obtained by surface treating one abrasive with another) may also be
employed. There are cases in which compounds or elements other than the
primary compound are contained in these abrasives; the effect does not
change so long as the content of the primary compound is equal to or
greater than 90 percent. The particle size of the abrasive is preferably
0.01 to 2 micrometers. To enhance electromagnetic characteristics, a
narrow particle size distribution is desirable. Abrasives of differing
particle size may be incorporated as needed to improve durability; the
same effect can be achieved with a single abrasive as with a wide
particle size distribution. It is preferable that the tap density is 0.3
to 2 g/cc, the moisture content is 0.1 to 5 percent, the pH is 2 to 11,
and the specific surface area is 1 to 30 m2/g. The shape of the
abrasive employed may be acicular, spherical, cubic, plate-shaped or the
like. However, a shape comprising an angular portion is desirable due to
high abrasiveness. Specific examples are AKP-12, AKP-15, AKP-20, AKP-30,
AKP-50, HIT-20, HIT-30, HIT-55, HIT-60, HIT-70, HIT-80, and HIT-100 made
by Sumitomo Chemical Co., Ltd.; ERC-DBM, HP-DBM, and HPS-DBM made by
Reynolds Corp.; WA10000 made by Fujimi Abrasive Corp.; UB20 made by
Uemura Kogyo Corp.; G-5, Chromex U2, and Chromex U1 made by Nippon
Chemical Industrial Co., Ltd.; TF100 and TF140 made by Toda Kogyo Corp.;
Beta Random Ultrafine made by Ibiden Co., Ltd.; and B-3 made by Showa
Kogyo Co., Ltd. These abrasives may be added as needed to the nonmagnetic
layer. Addition of abrasives to the nonmagnetic layer can be done to
control surface shape, control how the abrasive protrudes, and the like.
The particle size and quantity of the abrasives added to the magnetic
layer and nonmagnetic layer should be set to optimal values.

[0089]These organic solvents need not be 100 weight percent pure and may
contain impurities such as isomers, unreacted materials, by-products,
decomposition products, oxides and moisture in addition to the main
components. The content of these impurities is preferably equal to or
less than 30 weight percent, more preferably equal to or less than 10
weight percent. Preferably the same type of organic solvent is employed
in the magnetic layer and in the nonmagnetic layer. However, the amount
added may be varied. The stability of coating is increased by using a
solvent with a high surface tension (such as cyclohexanone or dioxane) in
the nonmagnetic layer. Specifically, it is important that the arithmetic
mean value of the magnetic layer solvent composition be not less than the
arithmetic mean value of the nonmagnetic layer solvent composition. To
improve dispersion properties, a solvent having a somewhat strong
polarity is desirable. It is desirable that solvents having a dielectric
constant equal to or higher than 15 are comprised equal to or higher than
50 percent of the solvent composition. Further, the dissolution parameter
is desirably 8 to 11.

[0090]The types and quantities of dispersing agents, lubricants, and
surfactants employed in the magnetic layer may differ from those employed
in the nonmagnetic layer, described further below, in the present
invention. For example (the present invention not being limited to the
embodiments given herein), a dispersing agent usually has the property of
adsorbing or bonding by means of a polar group. In the magnetic layer,
the dispersing agent adsorbs or bonds by means of the polar group
primarily to the surface of the ferromagnetic powder, and in the
nonmagnetic layer, primarily to the surface of the nonmagnetic powder. It
is surmised that once a cyclic compound has adsorbed or bonded, it tends
not to dislodge readily from the surface of a metal, metal compound, or
the like. Accordingly, the surface of a ferromagnetic powder or the
surface of a nonmagnetic powder becomes covered with the alicyclic ring,
aromatic ring, heterocyclic ring, and the like. This enhances the
compatibility of the ferromagnetic powder or nonmagnetic powder with the
binder resin component, further improving the dispersion stability of the
ferromagnetic powder or nonmagnetic powder. Further, lubricants are
normally present in a free state. Thus, it is conceivable to use fatty
acids with different melting points in the nonmagnetic layer and magnetic
layer to control seepage onto the surface, employ esters with different
boiling points and polarity to control seepage onto the surface, regulate
the quantity of the surfactant to enhance coating stability, and employ a
large quantity of lubricant in the nonmagnetic layer to enhance the
lubricating effect. All or some part of the additives employed in the
present invention can be added in any of the steps during the
manufacturing of coating liquids for the magnetic layer and nonmagnetic
layer. For example, there are cases where they are mixed with the
ferromagnetic powder prior to the kneading step; cases where they are
added during the step in which the ferromagnetic powder, binder, and
solvent are kneaded; cases where they are added during the dispersion
step; cases where they are added after dispersion; and cases where they
are added directly before coating.

Nonmagnetic Layer

[0091]Details of the nonmagnetic layer will be described below. In the
manufacturing method of the present invention, it is possible to form a
magnetic layer by coating a magnetic layer coating liquid directly on a
nonmagnetic support and drying the coating liquid. It is also possible to
manufacture a magnetic recording medium comprising a nonmagnetic layer
and a magnetic layer in this order on a nonmagnetic support by coating a
nonmagnetic layer coating liquid on a nonmagnetic support, and then
coating a magnetic layer coating liquid thereover and drying it. The
nonmagnetic layer coating liquid can comprise a nonmagnetic powder and a
binder, and optionally comprise additives. Both organic and inorganic
substances may be employed as the nonmagnetic powder in the nonmagnetic
layer coating liquid. Carbon black may also be employed. Examples of
inorganic substances are metals, metal oxides, metal carbonates, metal
sulfates, metal nitrides, metal carbides, and metal sulfides.

[0093]The nonmagnetic powder may be acicular, spherical, polyhedral, or
plate-shaped. The crystallite size of the nonmagnetic powder preferably
ranges from 4 nm to 500 nm, more preferably from 40 to 100 nm. A
crystallite size falling within a range of 4 nm to 500 nm is desirable in
that it facilitates dispersion and imparts a suitable surface roughness.
The average particle diameter of the nonmagnetic powder preferably ranges
from 5 nm to 500 nm. As needed, nonmagnetic powders of differing average
particle diameter may be combined; the same effect may be achieved by
broadening the average particle distribution of a single nonmagnetic
powder. The preferred average particle diameter of the nonmagnetic powder
ranges from 10 to 200 nm. Within a range of 5 nm to 500 nm, dispersion is
good and good surface roughness can be achieved.

[0094]The specific surface area of the nonmagnetic powder preferably
ranges from 1 to 150 m2/g, more preferably from 20 to 120 m2/g,
and further preferably from 50 to 100 m2/g. Within the specific
surface area ranging from 1 to 150 m2/g, suitable surface roughness
can be achieved and dispersion is possible with the desired quantity of
binder. Oil absorption capacity using dibutyl phthalate (DBP) preferably
ranges from 5 to 100 mL/100 g, more preferably from 10 to 80 mL/100 g,
and further preferably from 20 to 60 mL/100 g. The specific gravity
ranges from, for example, 1 to 12, preferably from 3 to 6. The tap
density ranges from, for example, 0.05 to 2 g/mL, preferably from 0.2 to
1.5 g/mL. A tap density falling within a range of 0.05 to 2 g/mL can
reduce the amount of scattering particles, thereby facilitating handling,
and tends to prevent solidification to the device. The pH of the
nonmagnetic powder preferably ranges from 2 to 11, more preferably from 6
to 9. When the pH falls within a range of 2 to 11, the coefficient of
friction does not become high at high temperature or high humidity or due
to the freeing of fatty acids. The moisture content of the nonmagnetic
powder ranges from, for example, 0.1 to 5 weight percent, preferably from
0.2 to 3 weight percent, and more preferably from 0.3 to 1.5 weight
percent. A moisture content falling within a range of 0.1 to 5 weight
percent is desirable because it can produce good dispersion and yield a
stable coating viscosity following dispersion. An ignition loss of equal
to or less than 20 weight percent is desirable and nonmagnetic powders
with low ignition losses are desirable.

[0095]When the nonmagnetic powder is an inorganic powder, the Mohs'
hardness is preferably 4 to 10. Durability can be ensured if the Mohs'
hardness ranges from 4 to 10. The stearic acid (SA) adsorption capacity
of the nonmagnetic powder preferably ranges from 1 to 20 μmol/m2,
more preferably from 2 to 15 μmol/m2. The heat of wetting in
25° C. water of the nonmagnetic powder is preferably within a
range of 200 to 600 erg/cm2 (approximately 200 to 600 mJ/m2). A
solvent with a heat of wetting within this range may also be employed.
The quantity of water molecules on the surface at 100 to 400° C.
suitably ranges from 1 to 10 pieces per 100 Angstroms. The pH of the
isoelectric point in water preferably ranges from 3 to 9. The surface of
these nonmagnetic powders is preferably treated with Al2O3,
SiO2, TiO2, ZrO2, SnO2, Sb2O3, and ZnO. The
surface-treating agents of preference with regard to dispersibility are
Al2O3, SiO2, TiO2, and ZrO2, and
Al2O3, SiO2 and ZrO2 are further preferable. They may
be employed singly or in combination. Depending on the objective, a
surface-treatment coating layer with a coprecipitated material may also
be employed, the coating structure which comprises a first alumina
coating and a second silica coating thereover or the reverse structure
thereof may also be adopted. Depending on the objective, the
surface-treatment coating layer may be a porous layer, with homogeneity
and density being generally desirable.

[0097]Carbon black may be combined with nonmagnetic powder in the
nomagnetic layer coating liquid to reduce surface resistivity, reduce
light transmittance, and achieve a desired micro-Vickers hardness in the
nonmagnetic layer. The micro-Vickers hardness of the nonmagnetic layer is
normally 25 to 60 kg/mm2 (approximately 245 to 588 MPa), desirably
30 to 50 kg/mm2 (approximately 294 to 490 MPa) to adjust head
contact. It can be measured with a thin film hardness meter (HMA-400 made
by NEC Corporation) using a diamond triangular needle with a tip radius
of 0.1 micrometer and an edge angle of 80 degrees as indenter tip.
"Techniques for evaluating thin-film mechanical characteristics," Realize
Corp., for details. The content of the above publication is expressly
incorporated herein by reference in its entirety. The light transmittance
is generally standardized to an infrared absorbance at a wavelength of
about 900 nm equal to or less than 3 percent. For example, in VHS
magnetic tapes, it has been standardized to equal to or less than 0.8
percent. To this end, furnace black for rubber, thermal black for rubber,
black for coloring, acetylene black and the like may be employed.

[0098]The specific surface area of the carbon black employed in the
nonmagnetic layer coating liquid is, for example, 100 to 500 m2/g,
preferably 150 to 400 m2/g. The DBP oil absorption capability is,
for example, 20 to 400 mL/100 g, preferably 30 to 200 mL/100 g. The
particle diameter of the carbon black is, for example, 5 to 80 nm,
preferably 10 to 50 nm, and more preferably, 10 to 40 nm. It is
preferable that the pH of the carbon black is 2 to 10, the moisture
content is 0.1 to 10 percent, and the tap density is 0.1 to 1 g/mL.

[0100]The carbon black employed may be surface-treated with a dispersant
or grafted with resin, or have a partially graphite-treated surface. The
carbon black may be dispersed in advance into the binder prior to
addition to the nonmagnetic coating liquid. These carbon blacks may be
used singly or in combination. When employing carbon black, the quantity
of the carbon black is preferably within a range not exceeding 50 weight
percent of the inorganic powder as well as not exceeding 40 weight
percent of the total weight of the nonmagnetic layer. For example, the
Carbon Black Handbook compiled by the Carbon Black Association, which is
expressly incorporated herein by reference in its entirety, may be
consulted for types of carbon black suitable for use in the nonmagnetic
layer.

[0101]Based on the objective, an organic powder may be added to the
nonmagnetic layer coating liquid. Examples of such an organic powder are
acrylic styrene resin powders, benzoguanamine resin powders, melamine
resin powders, and phthalocyanine pigments. Polyolefin resin powders,
polyester resin powders, polyamide resin powders, polyimide resin
powders, and polyfluoroethylene resins may also be employed. The
manufacturing methods described in Japanese Unexamined Patent Publication
(KOKAI) Showa Nos. 62-18564 and 60-255827 may be employed. The contents
of the above applications are expressly incorporated herein by reference
in their entirety.

[0102]Binders, lubricants, dispersing agents, additives, solvents,
dispersion methods, and the like suited to the magnetic layer may be
adopted to the nonmagnetic layer coating liquid. In particular, known
techniques for the quantity and type of binder and the quantity and type
of additives and dispersion agents employed in the magnetic layer may be
adopted thereto.

Nonmagnetic Support

[0103]Known films of the following may be employed as the nonmagnetic
support in the present invention: polyethylene terephthalate,
polyethylene naphthalate and other polyesters, polyolefins, cellulose
triacetate, polycarbonate, polyamides, polyimides, polyamidoimides,
polysulfones, aromatic polyamides, polybenzooxazoles and the like.
Supports having a glass transition temperature of equal to or higher than
100° C. are preferably employed. The use of polyethylene
naphthalate, aramid, or some other high-strength support is particularly
desirable. As needed, layered supports such as disclosed in Japanese
Unexamined Patent Publication (KOKAI) Heisei No. 3-224127, which is
expressly incorporated herein by reference in its entirety, may be
employed to vary the surface roughness of the magnetic surface and
support surface. These supports may be subjected beforehand to corona
discharge treatment, plasma treatment, adhesion enhancing treatment, heat
treatment, dust removal, and the like.

[0104]The center surface average surface roughness (SRa) of the support
measured with an optical interferotype surface roughness meter HD-2000
made by WYKO is preferably equal to or less than 8.0 nm, more preferably
equal to or less than 4.0 nm, further preferably equal to or less than
2.0 nm. Not only does such a support desirably have a low center surface
average surface roughness, but there are also desirably no large
protrusions equal to or higher than 0.5 μm. The surface roughness
shape may be freely controlled through the size and quantity of filler
added to the support as needed. Examples of such fillers are oxides and
carbonates of elements such as Ca, Si, and Ti, and organic fine powders
such as acrylic-based one. The support desirably has a maximum height
Rmax equal to or less than 1 μm, a ten-point average roughness
RZ equal to or less than 0.5 μm, a center surface peak height
RP equal to or less than 0.5 μm, a center surface valley depth
RV equal to or less than 0.5 μm, a center-surface surface area
percentage Sr of 10 percent to 90 percent, and an average wavelength
λa of 5 to 300 μm. To achieve desired electromagnetic
characteristics and durability, the surface protrusion distribution of
the support can be freely controlled with fillers. It is possible to
control within a range from 0 to 2,000 protrusions of 0.01 to 1 μm in
size per 0.1 mm2.

[0105]The F-5 value of the nonmagnetic support employed in the present
invention preferably ranges from 5 to 50 kg/mm2 (approximately 49 to
490 MPa). The thermal shrinkage rate of the support after 30 min at
100° C. is preferably equal to or less than 3 percent, more
preferably equal to or less than 1.5 percent. The thermal shrinkage rate
after 30 min at 80° C. is preferably equal to or less than 1
percent, more preferably equal to or less than 0.5 percent. The breaking
strength of the nonmagnetic support preferably ranges from 5 to 100
kg/mm2 (approximately 49 to 980 MPa). The modulus of elasticity
preferably ranges from 100 to 2,000 kg/mm2 (approximately 0.98 to
19.6 GPa). The thermal expansion coefficient preferably ranges from l- to
10-8/° C., more preferably from 10-5 to
10-6/° C. The moisture expansion coefficient is preferably
equal to or less than 10-4/RH percent, more preferably equal to or
less than 10-5/RH percent. These thermal characteristics,
dimensional characteristics, and mechanical strength characteristics are
desirably nearly equal, with a difference equal to less than 10 percent,
in all in-plane directions in the support.

[0106]An undercoating layer can be provided in the method of manufacturing
a magnetic recording medium of the present invention. Providing an
undercoating layer can enhance adhesive strength between the support and
the magnetic layer or nonmagnetic layer. For example, a polyester resin
that is soluble in solvent can be employed as the undercoating layer to
enhance adhesion. As described below, a smoothing layer can be provided
as an undercoating layer.

Layer Structure

[0107]In the magnetic recording medium manufactured by the manufacturing
method of the present invention, the thickness of the nonmagnetic support
preferably ranges from 3 to 80 micrometers, more preferably from 3 to 50
micrometers, further preferably from 3 to 10 micrometers. When an
undercoating layer is provided between the nonmagnetic support and the
nonmagnetic layer or the magnetic layer, the thickness of the
undercoating layer ranges from, for example, 0.01 to 0.8 micrometer,
preferably 0.02 to 0.6 micrometer.

[0108]An intermediate layer can be provided between the support and the
nonmagnetic layer or the magnetic layer and/or between the support and
the backcoat layer to improve smoothness. For example, the intermediate
layer can be formed by coating and drying a coating liquid comprising a
polymer on the surface of the nonmagnetic support, or by coating a
coating liquid comprising a compound (radiation-curable compound)
comprising intramolecular radiation-curable functional groups and then
irradiating it with radiation to cure the coating liquid.

[0109]A radiation-curable compound having a number average molecular
weight ranging from 200 to 2,000 is desirably employed. When the
molecular weight is within the above range, the relatively low molecular
weight can facilitate coating flow during the calendering step,
increasing moldability and permitting the formation of a smooth coating.

[0110]A radiation-curable compound in the form of a bifunctional acrylate
compound with the molecular weight of 200 to 2,000 is desirable.
Bisphenol A, bisphenol F, hydrogenated bisphenol A, hydrogenated
bisphenol F, and compounds obtained by adding acrylic acid or methacrylic
acid to alkylene oxide adducts of these compounds are preferred.

[0111]The radiation-curable compound can be used in combination with a
polymeric binder. Examples of the binder employed in combination are
conventionally known thermoplastic resins, thermosetting resins, reactive
resins, and mixtures thereof. When the radiation employed in the curing
process is UV radiation, a polymerization initiator is desirably employed
in combination. A known photoradical polymerization initiator,
photocationic polymerization initiator, photoamine generator, or the like
can be employed as the polymerization initiator.

[0112]A radiation-curable compound can also be employed in the nonmagnetic
layer.

[0113]The thickness of the magnetic layer can be optimized based on the
saturation magnetization of the head employed, the length of the head
gap, and the recording signal band, and is normally 10 to 150 nm,
preferably 20 to 120 nm, more preferably 30 to 100 nm, further preferably
30 to 80 nm. The thickness variation (σ/δ) in the magnetic
layer is preferably within ±50 percent, more preferably within ±30
percent. At least one magnetic layer is sufficient. The magnetic layer
may be divided into two or more layers having different magnetic
characteristics, and a known configuration relating to multilayered
magnetic layer may be applied.

[0114]The thickness of the nonmagnetic layer ranges from, for example, 0.1
to 3.0 μm, preferably 0.2 to 2.0 μm, and more preferably 0.3 to 1.5
μm. The nonmagnetic layer in the present invention is effective so
long as it is substantially nonmagnetic. For example, it exhibits the
effect of the present invention even when it comprises impurities or
trace amounts of magnetic material that have been intentionally
incorporated, and can be viewed as substantially having the same
configuration as the magnetic recording medium of the present invention.
The term "substantially nonmagnetic" is used to mean having a residual
magnetic flux density in the nonmagnetic layer of equal to or less than
10 mT, or a coercive force Hc of equal to or less than 7.96 kA/m (100
Oe), it being preferable not to have a residual magnetic flux density or
coercive force at all.

Backcoat Layer

[0115]A backcoat layer can be provided on the surface of the nonmagnetic
support, opposite to the surface on which the magnetic layer is provided.
The backcoat layer desirably comprises carbon black and inorganic powder.
The formula of the magnetic layer or nonmagnetic layer can be applied to
the binder and various additives of the backcoat layer. The formula of
the nonmagnetic layer is preferred. The backcoat layer is preferably
equal to or less than 0.9 micrometer, more preferably 0.1 to 0.7
micrometer, in thickness.

[0116]Details of the magnetic recording medium manufactured by the
manufacturing method of the present invention, such as preferred physical
properties, are as set forth below for the magnetic recording medium of
the present invention.

[0117]The method of manufacturing a magnetic recording medium of the
present invention will be described below through specific embodiments of
the detailed procedure.

[0118]The magnetic layer coating liquid employed in the manufacturing
method of the present invention comprises components A, B, and C. The
details of these components are as set forth above. The surface on which
the magnetic layer coating liquid is coated does not have to be the
surface of the nonmagnetic support; when manufacturing a magnetic
recording medium having a nonmagnetic layer, the magnetic layer coating
liquid can be directly or indirectly coated on the nonmagnetic layer.

[0119]The process for manufacturing coating liquids for forming magnetic,
nonmagnetic and backcoat layers comprises at least a kneading step, a
dispersing step, and a mixing step to be carried out, if necessary,
before and/or after the kneading and dispersing steps. Each of the
individual steps may be divided into two or more stages. All of the
starting materials employed in the present invention, including the
ferromagnetic powder, nonmagnetic powder, binders, carbon black,
abrasives, antistatic agents, lubricants, solvents, and the like, may be
added at the beginning of, or during, any of the steps. Moreover, the
individual starting materials may be divided up and added during two or
more steps. For example, polyurethane may be divided up and added in the
kneading step, the dispersion step, and the mixing step for viscosity
adjustment after dispersion. To achieve the object of the present
invention, conventionally known manufacturing techniques may be utilized
for some of the steps. A kneader having a strong kneading force, such as
an open kneader, continuous kneader, pressure kneader, or extruder is
preferably employed in the kneading step. Details of the kneading process
are described in Japanese Unexamined Patent Publication (KOKAI) Heisei
Nos. 1-106338 and 1-79274. The contents of these applications are
incorporated herein by reference in their entirety. Further, glass beads
may be employed to disperse the coating liquids for magnetic, nonmagnetic
and backcoat layers, with a dispersing medium with a high specific
gravity such as zirconia beads, titania beads, and steel beads being
suitable for use. The particle diameter and fill ratio of these
dispersing media can be optimized for use. A known dispersing device may
be employed.

[0120]For the addition of the above-described compound (component C) to be
effective, component C is desirably present at the stage where the
ferromagnetic powder and binder are brought into contact. This is to
prevent the binder from contacting the surface of the ferromagnetic
powder before component C has adhered to the surface of the ferromagnetic
powder. Accordingly, the magnetic layer coating liquid is desirably
prepared by simultaneously mixing component A (ferromagnetic powder),
component B (binder), and component C (cyclic compound), or by mixing
components A and C to obtain a mixture and then mixing component B to the
mixture. Preparation by mixing component B to a mixture obtained by
mixing components A and C is preferred. Mixing components A and C first
can allow a larger amount of component C to adsorb to the surface of the
ferromagnetic powder, inhibiting the generation of low-molecular-weight
components derived from the binder.

[0121]Components A, B, and C are desirably specifically mixed by the
following methods:

(1) Components A and C are dry dispersed for about 15 to 30 minutes in
advance, and then added to an organic solvent. Component B can be
simultaneously added with the dispersion, or can be added after the
dispersion.(2) Components A and C are dispersed for about 15 to 30
minutes in an organic solvent, and then dried. The dry mixture is
suitably comminuted and then added to an organic solvent. Component B can
be simultaneously added with the mixture, or added after the mixture.(3)
Components A and C are dispersed for about 15 to 30 minutes in an organic
solvent, after which component B is added.(4) Components A, B, and C are
simultaneously added to an organic solvent and dispersed.

[0122]In the process of manufacturing the magnetic recording medium, for
example, the nonmagnetic layer coating liquid is coated in a quantity
calculated to yield a coating of prescribed thickness on the surface of a
running nonmagnetic support to form the nonmagnetic layer, after which
the magnetic layer coating liquid is coated thereover in a quantity
calculated to yield a coating of prescribed thickness to form the
magnetic layer. Multiple magnetic layer coating liquids can be
successively or simultaneously coated in a multilayer coating, and the
nonmagnetic layer coating liquid and magnetic layer coating liquid can be
successively or simultaneously coated in a multilayer coating. The
coating apparatus used to coat the magnetic layer coating liquid or
nonmagnetic layer coating liquid can be an air doctor coater, blade
coater, rod coater, extrusion coater, air knife coater, squeeze coater,
impregnating coater, reverse roll coater, transfer roll coater, gravure
coater, kiss coater, cast coater, spray coater, spin coater, or the like.
Details of the coating apparatus are described in, for example, "The Most
Recent Coating Techniques," published by the Sogo Technology Center
(Ltd.) (May 31, 1983), which is expressly incorporated herein by
reference in its entirety.

[0123]As for a magnetic tape, the coating layer that is formed by applying
the magnetic layer coating liquid can be magnetic field orientation
processed using cobalt magnets or solenoids on the ferromagnetic powder
contained in the coating layer. As for a disk, an adequately isotropic
orientation can be achieved in some products without orientation using an
orientation device, but the use of a known random orientation device in
which cobalt magnets are alternately arranged diagonally, or alternating
fields are applied by solenoids, is desirable. In the case of
ferromagnetic metal powder, the term "isotropic orientation" generally
refers to a two-dimensional in-plane random orientation, which is
desirable, but can refer to a three-dimensional random orientation
achieved by imparting a perpendicular component. Further, a known method,
such as opposing magnets of opposite poles, can be employed to effect
perpendicular orientation, thereby imparting an isotropic magnetic
characteristic in the peripheral direction. Perpendicular orientation is
particularly desirable when conducting high-density recording. Spin
coating can be used to effect peripheral orientation.

[0124]The drying position of the coating is desirably controlled by
controlling the temperature and flow rate of drying air, and coating
speed. A coating speed of 20 m/min to 1,000 m/min and a dry air
temperature of equal to or higher than 60° C. are desirable.
Suitable predrying can be conducted prior to entry into the magnet zone.

[0125]The coated stock material thus obtained can be temporarily wound on
a take-up roll, and then unwound from the take-up roll and calendered.

[0126]For example, super calender rolls can be employed in calendering.
Calendering can enhance surface smoothness, eliminate voids produced by
the removal of solvent during drying, and increase the fill rate of the
ferromagnetic powder in the magnetic layer, thus yielding a magnetic
recording medium of good electromagnetic characteristics. The calendering
step is desirably conducted by varying the calendering conditions in
response to the smoothness of the surface of the coated stock material.

[0127]The surface smoothness of the coated stock material can be
controlled by controlling the calender roll temperature, calender roll
speed, and calender roll tension. Taking into account the properties of a
particulate medium, it is desirable to control the surface smoothness by
means of the calender roll pressure and calender roll temperature.
Generally, the calender roll pressure is reduced, or the calender roll
temperature is lowered, to diminish the surface smoothness of the final
product. Conversely, the calender roll pressure can be increased or the
calender roll temperature can be raised to increase the surface
smoothness of the final product.

[0128]Alternatively, the magnetic recording medium following the
calendering step can be thermally processed to induce thermosetting. Such
thermal processing can be suitably determined based on the blending
formula of the magnetic layer coating liquid. The thermal processing
temperature is, for example, 35 to 100° C., desirably 50 to
80° C. The thermal processing time is, for example, 12 to 72
hours, desirably 24 to 48 hours.

[0129]Rolls of a heat-resistant plastic such as epoxy, polyimide,
polyamide, or polyamidoimide, can be employed as the calender rolls.
Processing with metal rolls is also possible.

[0130]As for the calendaring conditions, the calender roll temperature
ranges from, for example, 60 to 100° C., preferably 70 to
100° C., and more preferably 80 to 100° C. The pressure
ranges from, for example, 100 to 500 kg/cm (98 to 490 kN/m), preferably
200 to 450 kg/cm (196 to 441 kN/m), and more preferably 300 to 400 kg/cm
(294 to 392 kN/m). To improve surface smoothness of the magnetic layer,
the nonmagnetic layer surface can be calendered. Calendering for the
nonmagnetic layer is preferably conducted under the above-described
conditions.

[0131]The magnetic recording medium obtained can be cut to desired size
with a cutter or the like. The cutter is not specifically limited, but
desirably comprises multiple sets of a rotating upper blade (male blade)
and lower blade (female blade). The slitting speed, engaging depth,
peripheral speed ratio of the upper blade (male blade) and lower blade
(female blade) (upper blade peripheral speed/lower blade peripheral
speed), period of continuous use of slitting blade, and the like are
suitably selected.

Magnetic Recording Medium

[0132]The present invention further relates to a magnetic recording medium
comprising a magnetic layer comprising a ferromagnetic powder and a
binder on a nonmagnetic support. The magnetic recording medium of the
present invention is manufactured by the manufacturing method of the
present invention. Details of the magnetic recording medium of the
present invention, such as various components comprised and preferred
physical properties of various layers, are as set forth above.

[0133]The physical properties of the magnetic recording medium of the
present invention will be described below.

Physical Properties

[0134]The coercivity (Hc) of the magnetic layer is preferably 143.2 to
318.3 kA/m (approximately 1800 to 4000 Oe), more preferably 159.2 to
278.5 kA/m (approximately 2000 to 3500 Oe). Narrower coercivity
distribution is preferable. The SFD and SFDr are preferably equal to or
lower than 0.8, more preferably equal to or lower than 0.5.

[0135]The coefficient of friction of the magnetic recording medium
relative to the head is, for example, equal to or less than 0.5 and
preferably equal to or less than 0.3 at temperatures ranging from
-10° C. to 40° C. and humidity ranging from 0 percent to 95
percent, the surface resistivity on the magnetic surface preferably
ranges from 104 to 108 ohm/sq, and the charge potential
preferably ranges from -500 V to +500 V. The modulus of elasticity at 0.5
percent extension of the magnetic layer preferably ranges from 0.98 to
19.6 GPa (approximately 100 to 2,000 kg/mm2) in each in-plane
direction. The breaking strength preferably ranges from 98 to 686 MPa
(approximately 10 to 70 kg/mm2). The modulus of elasticity of the
magnetic recording medium preferably ranges from 0.98 to 14.7 GPa
(approximately 100 to 1500 kg/mm2) in each in-plane direction. The
residual elongation is preferably equal to or less than 0.5 percent, and
the thermal shrinkage rate at all temperatures below 100° C. is
preferably equal to or less than 1 percent, more preferably equal to or
less than 0.5 percent, and most preferably equal to or less than 0.1
percent.

[0136]The glass transition temperature (i.e., the temperature at which the
loss elastic modulus of dynamic viscoelasticity peaks as measured at 110
Hz with a dynamic viscoelastometer, such as RHEOVIBRON made by A&D Co.
Ltd) of the magnetic layer preferably ranges from 50 to 180° C.,
and that of the nonmagnetic layer preferably ranges from 0 to 180°
C. The loss elastic modulus preferably falls within a range of
1×107 to 8×108 Pa (approximately 1×108
to 8×109 dyne/cm2) and the loss tangent is preferably
equal to or less than 0.2. Adhesion failure tends to occur when the loss
tangent becomes excessively large. These thermal characteristics and
mechanical characteristics are desirably nearly identical, varying by
equal to or less than 10 percent, in each in-plane direction of the
medium.

[0137]The residual solvent contained in the magnetic layer is preferably
equal to or less than 100 mg/m2 and more preferably equal to or less
than 10 mg/m2. The void ratio in the coated layers, including both
the nonmagnetic layer and the magnetic layer, is preferably equal to or
less than 40 volume percent, more preferably equal to or less than 30
volume percent. Although a low void ratio is preferable for attaining
high output, there are some cases in which it is better to ensure a
certain level based on the object. For example, in many cases, larger
void ratio permits preferred running durability in disk media in which
repeat use is important.

[0138]Physical properties of the nonmagnetic layer and magnetic layer may
be varied based on the objective in the magnetic recording medium of the
present invention. For example, the modulus of elasticity of the magnetic
layer may be increased to improve running durability while simultaneously
employing a lower modulus of elasticity than that of the magnetic layer
in the nonmagnetic layer to improve the head contact of the magnetic
recording medium.

EXAMPLES

[0139]The present invention will be described in detail below based on
examples. However, the present invention is not limited to the examples.
The term "parts" given in Examples are weight parts unless specifically
stated otherwise.

[0141]The various components of the above nonmagnetic layer coating liquid
were first kneaded in an open kneader and then dispersed in a sand mill.
Five parts of polyisocyanate (Coronate L, made by Nippon Polyurethane
Industry Co., Ltd.) were added to the dispersion obtained, 40 parts of a
mixed solvent of methyl ethyl ketone and cyclohexanone were further
added, and the mixture was mixed and stirred. The mixture was then
filtered with a filter having a pore diameter of 1 micrometer to prepare
the nonmagnetic layer coating liquid.

[0142]The magnetic layer coating liquid was prepared as follows. The
hexagonal ferrite powder and 1-naphthoic acid were dry dispersed for 15
minutes, the dispersion was kneaded with the above-listed magnetic layer
components in an open kneader, and the mixture was dispersed in a sand
mill. Three parts of polyisocyanate (Coronate L, made by Nippon
Polyurethane Industry Co., Ltd.) were added to the dispersion obtained,
40 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone were
further added, and the mixture was mixed and stirred. The mixture was
then filtered with a filter having a pore diameter of 1 micrometer to
obtain the magnetic layer coating liquid.

[0143]The backcoat layer coating liquid was prepared as follows. The
above-listed components were kneaded in a continuous kneader and
dispersed in a sand mill. To the dispersion obtained were added 40 parts
of polyisocyanate (Coronate L, made by Nippon Polyurethane Industry Co.,
Ltd.) and 1,000 parts of methyl ethyl ketone. The mixture was stirred and
then filtered with a filter having a pore diameter of 1 micrometer.

[0144]The nonmagnetic layer coating liquid was coated in a quantity
calculated to yield a nonmagnetic layer with a dry thickness of 1.5
micrometers and the magnetic layer coating liquid was coated in a
quantity calculated to yield a magnetic layer with a dry thickness of
0.10 micrometer on a support (biaxially-drawn polyethylene terephthalate)
7 micrometer in thickness in such a manner as to obtain a total dry tape
thickness of 8.6 micrometers in a simultaneous multilayer coating, and
the coating liquids were dried. Subsequently, the backcoat layer coating
liquid was coated to the opposite surface from the magnetic layer surface
in a quantity calculated to yield a backcoat layer with a dry thickness
of 0.5 micrometer.

[0145]Subsequently, the medium was calendered with a seven-stage calender
comprised of only metal rolls at a rate of 100 m/min, a linear pressure
of 350 kg/cm (343 kN/m), and a temperature of 80° C. The roll
obtained was heat treated for 48 hours at 50° C. Next, the medium
was slit to 1/2 inch width to prepare a magnetic tape.

[0146]With the exceptions that the polyurethane resin contained in the
magnetic layer coating liquid and nonmagnetic layer coating liquid was
replaced with the polyurethane resin having the weight average molecular
weight, polar group type, and polar group quantity indicated in Table 1,
magnetic tapes were manufactured by the same method as in Example 1.

Examples 6 to 13, 16 to 20, and Comparative Example 2

[0147]With the exception that component C, and/or the quantity thereof,
contained in the magnetic layer coating liquid was changed as indicated
in Table 1, magnetic tapes were manufactured by the same method as in
Example 1.

Examples 21 and 22

[0148]With the exception that the hexagonal ferrite contained in the
magnetic layer coating liquid was changed to the hexagonal ferrite having
the average plate diameter shown in Table 1, magnetic tapes were
manufactured by the same method as in Example 1.

Example 28

[0149]A magnetic tape was manufactured by the same method as in Example 1,
with the exceptions that the hexagonal ferrite powder contained in the
magnetic layer coating liquid was changed to a ferromagnetic metal powder
having the average major axis length shown in Table 1, the compound shown
in Table 1 was employed as component C, the magnetic layer and the
nonmagnetic layer were oriented by cobalt magnets having a magnetic force
of 0.3 T (3,000 G) and solenoids having a magnetic force of 0.15 T (1,500
G) while the magnetic layer and nonmagnetic layer were still wet during
the course of forming the magnetic layer (simultaneous multilayer
coating) and then dried, and a backcoat layer was coated in a quantity
calculated to yield a dry thickness of 0.5 micrometer.

Example 29

[0150]With the exceptions that hexagonal ferrite powder, binder, and
1-naphthoic acid were simultaneously dispersed during the preparation of
the magnetic layer coating liquid, a magnetic tape was manufactured by
the same method as in Example 1.

Comparative Example 3

[0151]With the exception that component C was not added to the magnetic
layer coating liquid, a magnetic tape was manufactured by the same method
as in Example 1.

Comparative Example 4

[0152]With the exceptions that the polyurethane resin contained in the
magnetic layer coating liquid and nonmagnetic layer coating liquid was
replaced with the polyurethane resin having the weight average molecular
weight, type of polar group, and quantity of polar group shown in Table
1, and component C was not added, a magnetic tape was manufactured by the
same method as in Example 1.

Comparative Examples 7 and 8

[0153]With the exception that the hexagonal ferrite contained in the
magnetic layer coating liquid was replaced with the hexagonal ferrite
having the average plate diameter shown in Table 1, magnetic tapes were
manufactured by the same method as in Example 1.

Examples 26 and 27, Comparative Examples 11 and 12

[0154]With the exception that the hexagonal ferrite contained in the
magnetic layer coating liquid was replaced with the hexagonal ferrite
having the water content shown in Table 1, magnetic tapes were
manufactured by the same method as in Example 1.

1. The Magnetic Layer Surface Roughness

[0155]The magnetic layer surface roughness was measured under the
following conditions:

[0157]Employing a magnetic tape tester, a tape sample 800 m in length per
roll was run at a running speed of 6 m/s, a back tension of 0.7 N, and a
tape/head angle (1/2 of the lap angle) of 10 degrees while winding/taking
up the tape between two reels.

[0158]Employing a linear head, a 19.0 MHz (linear recording density of 160
kfci) signal was recorded and reproduced while running the tape sample in
the above-described "Running method." The reproduction signal was
inputted to an R3361C made by Advantest Corp., the peak signal of 19.0
MHz was adopted as the signal output (S), and the integral noise (N) was
measured over the range of 1 to 37.7 MHz, excluding 19.0 MHz±0.3 MHz.
The ratio was adopted as the S/N ratio. A value of equal to or higher
than 20 dB was considered to indicate good electromagnetic
characteristics.

3. Head Grime

[0159]A tape sample was run 500 m in the above-described "Running method."
After running the tape, the head was examined by optical microscope and
the head grime was evaluated. The image of the head that was examined by
optical microscope was input into a PC and binary processed. (The head
was observed at a magnification of 50.) A surface ratio of the tape
sliding surface of the head that was 0 to 5 percent covered with grime
was evaluated as "Excellent," more than 5 percent but equal to or less
than 15 percent as "good," and more than 15 percent as "X."

[0160]Examples 1 to 29 produced smooth magnetic layers and exhibited good
electromagnetic characteristics. Despite the good surface property of the
magnetic layer, no head grime was observed.

[0161]In Comparative Example 1, in which a binder that did not contain the
prescribed polar group was employed, the surface of the magnetic layer
was rough and good electromagnetic characteristics could not be achieved.

[0162]In Comparative Example 2, in which the compound having a carboxyl
group and/or a hydroxyl group serving as a surface modifying agent was
replaced with a cyclic compound (aniline) having neither a carboxyl group
nor a hydroxyl group, there was inadequate dispersion of the magnetic
layer and the smoothness of the magnetic layer surface decreased,
resulting in diminished electromagnetic characteristics.

[0163]In Comparative Example 3, in which no surface modifying agent was
added, head grime occurred. This was attributed to severing of the binder
through contact with the magnetic material, resulting in the presence of
a large quantity of low-molecular-weight compounds on the surface of the
magnetic layer.

[0164]In Comparative Examples 4 and 5, in which few polar groups were
present in the binder, the surface of the magnetic layer was rough and
the electromagnetic characteristics deteriorated. Conversely, in
Comparative Example 6, in which the quantity of polar groups in the
binder was excessive, the electromagnetic characteristics deteriorated.

[0165]In Comparative Example 7, the particle diameter of the ferromagnetic
powder was excessively small, and, as set forth above, the bonds between
magnetic particles weakened, the coating strength of the magnetic layer
diminished, and the coating separated to a degree precluding evaluation
of the S/N ratio. By contrast, in Comparative Example 8, in which the
particle diameter of the ferromagnetic powder was excessively large, the
electromagnetic characteristics deteriorated.

[0166]In Comparative Example 9, in which the weight average molecular
weight of the binder was low, head grime occurred. This was attributed to
the presence of a large number of low-molecular-weight compounds on the
surface of the magnetic layer.

[0167]In Comparative Example 10, in which the weight average molecular
weight of the binder exceeded 200,000, the dispersion of the magnetic
layer was inadequate and the electromagnetic characteristics
deteriorated.

[0168]In Comparative Example 11, the moisture content of the magnetic
material was low, the magnetic layer surface was rough, and the
electromagnetic characteristics deteriorated. This was attributed to the
binder not being able to adequately adsorb to the magnetic material. In
Comparative Example 12, in which the moisture content of the magnetic
material was excessive, the magnetic layer surface was rough and the
electromagnetic characteristics deteriorated. This was attributed to the
high moisture content causing the reaction with the polyisocyanate in the
magnetic layer coating liquid to advance excessively, resulting in a
rough magnetic layer surface.

[0169]The magnetic recording medium of the present invention is suitable
as a magnetic recording medium for high-density recording.

[0170]Although the present invention has been described in considerable
detail with regard to certain versions thereof, other versions are
possible, and alterations, permutations and equivalents of the version
shown will become apparent to those skilled in the art upon a reading of
the specification and study of the drawings. Also, the various features
of the versions herein can be combined in various ways to provide
additional versions of the present invention. Furthermore, certain
terminology has been used for the purposes of descriptive clarity, and
not to limit the present invention. Therefore, any appended claims should
not be limited to the description of the preferred versions contained
herein and should include all such alterations, permutations, and
equivalents as fall within the true spirit and scope of the present
invention.

[0171]Having now fully described this invention, it will be understood to
those of ordinary skill in the art that the methods of the present
invention can be carried out with a wide and equivalent range of
conditions, formulations, and other parameters without departing from the
scope of the invention or any embodiments thereof.

[0172]All patents and publications cited herein are hereby fully
incorporated by reference in their entirety. The citation of any
publication is for its disclosure prior to the filing date and should not
be construed as an admission that such publication is prior art or that
the present invention is not entitled to antedate such publication by
virtue of prior invention.

[0173]Unless otherwise stated, a reference to a compound or component
includes the compound or component by itself, as well as in combination
with other compounds or components, such as mixtures of compounds.

[0174]As used herein, the singular forms "a," "an," and "the" include the
plural reference unless the context clearly dictates otherwise.

[0175]Except where otherwise indicated, all numbers expressing quantities
of ingredients, reaction conditions, and so forth used in the
specification and claims are to be understood as being modified in all
instances by the term "about." Accordingly, unless indicated to the
contrary, the numerical parameters set forth in the following
specification and attached claims are approximations that may vary
depending upon the desired properties sought to be obtained by the
present invention. At the very least, and not to be considered as an
attempt to limit the application of the doctrine of equivalents to the
scope of the claims, each numerical parameter should be construed in
light of the number of significant digits and ordinary rounding
conventions.

[0176]Additionally, the recitation of numerical ranges within this
specification is considered to be a disclosure of all numerical values
and ranges within that range. For example, if a range is from about 1 to
about 50, it is deemed to include, for example, 1, 7, 34, 46.1, 23.7, or
any other value or range within the range.